You can apply a filter to the output of the media insert url form using the filter named wp_media_insert_url_form
:
add_filter('wp_media_insert_url_form', 'ex46632_media_insert_url_form');
function ex46632_media_insert_url_form($html) {
$html = str_replace('<input id="src" name="src" value=""', '<input id="src" name="src" value="http://mysite.com/images/"', $html);
return $html;
}
Something like this could be added to your theme’s functions.php
file.
This is untested code that is just as an example of direction.